During the past two years, the groundwork has been laid for studies that are either ongoing or to be initiated in the coming year. These concern the following questions: (1) How much, if any, infectivity might be present in blood and blood products to which individuals either having or incubating Creutzfeldt-Jakob disease (CJD) have contributed; (2) can very low levels of infectivity be correlated to the detection of prion protein; and (3) how much, if any, infectivity remains after exposures of infectious tissues to a range of temperatures between 300 and 1000 degrees centigrade.We have now completed studies of the levels of endogenous infectivity in blood and plasma fractions from mice infected with a human strain of CJD. A critical parallel experiment is currently under way in primates with a view to observing the presence or absence of infectivity in a species more closely related to humans. The question of whether blood infectivity in new variant CJD is similar to that in classic disease is also under investigation both in mice and primates. In the course of these experiments, a serious question has arisen as to whether infectivity in blood can be diluted out (e.g., in plasma pools of 100,000 instead of 10,000 donors). To answer this question, we are planning an experiment using endogenously infected mouse plasma, which contains about 20 infectious units per ml, and buffy coat, which contains about 100 infectious units per ml. Samples of each specimen will be diluted in several log 2 steps, and the total dilution volumes will be inoculated into healthy assay mice, which will be observed for one year. The search for a test sensitive enough to detect the very low levels of prion protein (PrP) that might accompany the presence of equally low levels of infectivity in the blood of humans incubating CJD is the subject of active research in a number of laboratories. For the past year, we have been collecting and processing blood samples from normal individuals and patients with CJD and other dementing illnesses, with a view to testing them for the presence of PrP using a comparatively new method combining competitive antigen-antibody inhibition and a laser driven capillary electrophoresis method. We are only just now beginning standardization studies to define the range of normal, and will be doing much more testing during the year. The other principle aspect of infectivity that we are studying is the heat resistance of CJD-like agents. Many years ago, we found that a hamster- adapted strain of scrapie was not entirely destoyed by heating at a temperature of 360 degrees centigrade. This surprising result was generally explained away by the fact that the sample had been freeze- dried under vacuum and was therefore protected both by dehydration and the absence of oxygen. We have recently repeated the experiment using the same scrapie strain in a macerate of fresh, untreated brain, in an open crucible, and have again found that a substantial residue of infectivity persists after heating 15 min at 300 degrees. We are therefore conducting new experiments with more narrowly spaced temperature increments to obtain a dose-response curve. We have also initiated experiments mimicking incineration conditions at 600 and 1000 degrees, in which we have collected both the ashed samples and trapped exhaust gases, both of which will be assayed for infectivity by inoculation of healthy hamsters. The results of these heat resistance experiments have obvious theoretical and practical consequences. - TSE, blood, infectivity, heat resistance, dilution - Human Subjects